1. Field of the Invention
The present invention relates to a hollow rack shaft for an automobile steering system, particularly relates to a rack shaft commonly known as dual pinion-type steering system, which is provided with two rack teeth groups on one rack shaft and its manufacturing method.
2. Description of the Related Arts
For an automobile steering system, a type of a rack shaft provided with two rack teeth groups on one rack shaft is known. Conventionally, this type of rack shaft is produced by the method as using a solid bar and forming rack teeth groups in two places of this bar.
Currently, to reduce the consumption of automobile fuel in consideration of an environmental problem and others, lightweightizing automobile parts is promoted. In such a situation, a dual pinion-type rack shaft is lightweightized by the method that deep holes are drilled to the solid body to the axial direction from the both ends of the shaft. However, the areas that can be machined by the above method are only the shaft areas from both ends to rack areas, which aren""t provided with rack teeth. Hence, the rack areas, and the shaft area between one rack area and the other rack area still remain solid, therefore lightweightizing is insufficient.
As disclosed in Japanese Patent Application Laid-Open No. Hei 11-180318 or No. Hei 11-278287, it is also proposed that a hollow rack shaft provided with a single rack teeth group is formed with one plate workpiece. However, such a manufacturing method is not developed for the dual pinion-type rack shaft and the fact is that the dual pinion-type rack shaft is still made of solid workpiece.
As described above, as the dual pinion-type rack shaft is made of a solid bar, it has the following problems.
1. Working time in a deep hole working process for lightweightizing is long.
2. Much material is wasted and a yield is lowered. Therefore, the manufacturing cost is increased.
3. Depth of each hole for lightweightizing the shaft is limited within a range of drilling from each end of the shaft.
4. As the rack areas and the area between them are solid, the rack shaft is not completely hollowed.
5. Therefore, lightweightizing of the rack shaft is insufficient.
The present invention has an object of solving the above-mentioned problems.
An object of the invention is to provide a hollow rack shaft for a steering system, having two rack teeth groups located longitudinally apart from each other, these rack groups are formed by plastic working.
The hollow rack shaft provided by the present invention is acquired by plastically deforming one plate workpiece.
Further, the respective rack teeth groups have phase difference as to an angle around the axis of the hollow rack shaft.
Further, in one embodiment, the rack teeth groups of the shaft are sequentially formed by die forming.
Further, in another embodiment, the rack teeth groups of the shaft are simultaneously formed by die forming.
Another object of the invention is to provide a method of manufacturing a hollow rack shaft for a steering system having two rack teeth groups, as located longitudinally apart from each other and as have phase difference as to an angle around axis of the shaft, wherein the shaft is formed of one plate workpiece which has two areas as the center lines of longitudinal direction at the areas are offset corresponding to the phases of the rack teeth groups.
Hereby, the following effects are expected by the hollow rack shaft according to the invention or its manufacturing method according to the invention.
1. Deep hole working process for lightweightizing is not required and working time is shortened.
2. As cutting process is not required, the material is hardly wasted and a yield rate is satisfactory.
3. The rack shaft is hollowed throughout the whole body.
4. Therefore, the rack shaft is lightweightized.
5. As the rack teeth groups are formed in different phase, a steering system ray-out can be designed flexibly. As each rack teeth group is formed with dies, the tooth profile can be designed hardly with restraint in consideration of working and the hollow rack shaft according to the invention can be coped with both constant gear ratio (CGR) and variable gear ratio (VGR).
6. The manufacturing process can be reduced by integrally forming a plate workpiece.
7. According to the invention, the manufacturing cost for the rack shaft can be reduced.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.